Organic matter (OM) is present in most terrestrial environments and is often found co-precipitated with ferrihydrite (Fh). Sorption or co-precipitation of OM with Fe oxides has been proposed to be an important mechanism for long-term C preservation. However, little is known about the impact of co-precipitated OM on reductive dissolution and transformation of Fe(III) (oxyhydr)oxides. Thus, we study the effect of humic acid (HA) co-precipitation on Fh reduction and secondary mineral formation by the dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens strain CN32. Despite similar crystal structure for all co-precipitates investigated, resembling 2-line Fh, the presence of co-precipitated HA resulted in lower specific surface areas. In terms of reactivity, co-precipitated HA resulted in slower Fh bioreduction rates at low C/Fe ratios (i.e., C/Fe ≤ 0.8), while high C/Fe ratios (i.e., C/Fe ≥ 1.8) enhanced the extent of bioreduction compared to pure Fh. The co-precipitated HA also altered the secondary Fe mineralization pathway by inhibiting goethite formation, reducing the amount of magnetite formation, and increasing the formation of a green rust-like phase. This study indicates that co-precipitated OM may influence the rates, pathway, and mineralogy of biogeochemical Fe cycling and anaerobic Fe respiration within soils.

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